1. Field of the Invention
The present invention relates to a vehicle body frame hollow member for absorbing impact energy generated when a vehicle collides through compression deformation.
2. Description of the Related Art
An applicant for the present invention proposed in the specification of Japanese Patent Publication No. JP-A-11-208519 a vehicle body frame construction in which a vehicle body frame is constructed by hollow extruded members the cross-sectional shape of an external contour of which is formed substantially into a polygon and which each have ribs connecting respective apical angles and a center shaft and extending in an axial direction thereof, whereby impact energy generated when a vehicle collides is absorbed through compression buckling deformation generated in the hollow extruded members when they receive impact load.
According to this construction, since a cross section can be set to provide a suitable buckling strength by setting a relationship between the length of a side of the polygon and the thickness thereof, any desirable impact energy absorbing properties can be obtained without enlarging the exterior dimensions of the hollow member excessively.
However, it is preferable that the energy absorbing amount relative to the cross section of a frame member be as large as possible, and in a case where a vehicle is such that a sufficient crushable zone cannot be set, a further increase in energy absorbing efficiency by the frame member is being desired.
The invention was made in view of this known situation, and a primary object thereof is to provide a vehicle body frame hollow member which can increase further the energy absorbing efficiency.
To solve the above object, according to a first aspect of the invention, there is provided a vehicle body frame hollow member having an external contour whose cross section forms a polygon, comprising: a plurality of plates, each defining a circumferential wall of the hollow member, each of points of intersection being formed by intersecting prolongations from exterior surfaces of the adjacent two plates; ribs connecting respective apical angles of the polygon with a center shaft thereof; and an elongated projection projecting outwardly of a circle tangent to the points of intersection and extending along an edge of each of the apical angles. Accordingly, the sectional area of the apical angles 3 can be increased without having to deteriorate the buckling deformation behaviors at the time of compression.
Further, the inventor eventually came to know that in a vehicle body frame hollow member having an external contour whose cross section substantially forms a regular polygon and comprising ribs which connect apical angles of respective apical angle portions of the regular polygon with a center shaft thereof and in which interior portions of an intersectional portion formed by two plates constituting an outer circumferential wall of the hollow member and the rib are each formed into a round shape, the relationship between a distance h in a rib-extending direction between an imaginary apex of an inner circumferential wall of the hollow member which shifts from an apex on the outer circumferential wall at the intersectional portion toward the center of the regular polygon by the thickness of the plate and a rib-side end point of the round-shaped portion and a distance L between plate-side end points of the round-shaped portions formed at ends of the plate constituting the outer circumferential wall remarkably influences the energy absorbing efficiency.
With a view to attaining the object, according to a second aspect of the invention, there is provided a vehicle body frame hollow member having an external contour whose cross section substantially forms a regular polygon and comprising ribs which connect apical angles of respective apical angle portions of the regular polygon with a center shaft thereof, the vehicle body frame hollow member being characterized in that interior portions of an intersectional portion formed by two plates constituting an outer circumferential wall of the hollow member and the rib are each formed into a radius, and in that a distance h in a rib-extending direction between an imaginary apex of an inner circumferential wall of the hollow member which shifts from an apex on the outer circumferential wall at the intersectional portion toward the center of the regular polygon by the thickness of the plate and a rib-side end point of the radius is equal to or greater than 13% of a distance L between plate-side end points of the radii formed at ends of the plate constituting the outer circumferential wall. According to the construction, of the entire cross section of the hollow member the cross section of the apical angle portions which provide a large plastic deformation amount can be attempted to be enhanced efficiently, whereby the energy absorbing efficiency can be improved remarkably as shown in FIG. 11.